The manufacture of semiconductor wafers involves several chemical treatment steps, including some wet processing steps. These wet processing steps are typically followed by liquid rinsing and drying of the wafers. There are known numerous devices and methods for using vapor processing to dry the wafers after the liquid rinsing steps.
In one class of prior methods, a vapor processing zone is formed over a reservoir of continuously boiling isopropyl alcohol ("IPA"). This vapor processing zone is contained by walls which enclose the four sides and by the reservoir at the bottom. A cooling coil is provided at the top of the processing zone. The substrates are loaded into the processing zone through the cooling coils on an elevator transport arm which moves them down through the cooling coils into the vapor processing zone for the required drying time. The substrates are then withdrawn back up through the cooling coils. A variation on this top loading method involves side loading and horizontal transport of the substrates through the vapor zone. Both approaches, however, utilize the common concept of moving the substrates through an active vapor processing zone produced by an adjacent reservoir of continuously boiling liquid.
This boiling reservoir-generated vapor processing zone approach has several significant drawbacks. Since the amount of vapor discharged to the atmosphere from the processing zone is a function of the cooling coil design, there are potential environmental control problems caused by the open areas through which the substrates must pass into and out of the cooling coils. The vapors passing through these open areas do not contact the cooling coils, and are therefore discharged to the atmosphere. There are also substantial safety hazards resulting from IPA's high flammability and explosiveness. Some of the existing devices utilize plate heaters operating at temperatures over 150 degrees Centigrade, well into the critical range for IPA flammability (flashpoint .about.12.degree.-21.degree. C.) and explosiveness (autoignition point .about.310.degree. C.). It is also the prior art practice to utilize quartz vessels for vapor generation. The fragility of these vessels and the proximity of the solvent to the heaters makes these processes inherently dangerous.
The mechanical substrate transport system described above adds further complexity to the drying process, and provides another potential contamination source in a demanding "clean" environment.
Another significant operating problem with this boiling reservoir approach is that the amount of vapor present in the vapor processing zone is an equilibrium phenomenon. As the cool substrates are lowered into the vapor processing zone, and vapors condense on the substrate surfaces, the vapor cloud initially present in the processing zone essentially collapses. It then takes time for the boiling reservoir to rebuild the "collapsed" vapor cloud. We refer to this phenomenon as "bottom-up vapor processing." This bottom-up vapor processing is problematic, even more so with hydrophilic wafers and patterned wafers. These wafers tend to hold water on their surface, and only have the bottom portion of water cleansed from the wafer surface upon initial descent into the vapor zone because the vapor cloud collapses in response to the cool wafers and cool wafer carrier which act like cooling coils. There follows a period of vapor cloud rebuilding which occurs as the wafers heat up from the condensing IPA vapors. As the vapor cloud recovers from the bottom up, droplets of water are absorbed by the IPA which dilute and/or contaminate the IPA which wets the lower portion of the wafers. It is also possible that micro-droplets of water will dry on the top portions of the wafer before the recovering vapor cloud envelopes the wafers. This results in undesirable water residue spots. This condition is aggravated by the reduced condensation rate on the upper portion of the wafers as they heat up.
Previous investigators' attempts to overcome these serious drawbacks with the boiling reservoir-generated vapor zone are not without their own problems. For example, efforts to reduce the collapsed vapor cloud recovery time by increasing the boiling rate of the IPA reservoir also increase the entrainment of impurities in the IPA. Efforts to reduce the collapsed vapor cloud recovery period by heating the sidewalls of the vapor processing zone tend toward aggravation of the previously described IPA explosive hazards. It is also apparent that the continuous vapor formation by the boiling reservoir and vapor condensation by the cooling coils between and during dryer cycles is unnecessarily energy intensive and wasteful.
In a different prior approach to vapor phase drying, the IPA vapor is generated external to the vapor processing zone and to the dryer itself, and then imported to the processing zone. This method has a number of drawbacks, the most significant of which is the inability of this approach to provide an adequate supply of pure vapor to the processing zone in a uniform manner to process a batch of substrates. The generation of vapor at a remote source is followed by transport of the vapor through a tube to the processing area. This tube can undesirably restrict the amount of vapor which is available for processing. Since uniform vapor coverage is critical to the prevention of premature water microdroplet drying these vapor transport limitations adversely effect the process performance. When the IPA boiling rate is increased in an attempt to increase vapor density and to provide uniform coverage, it results in increased impurities concentration in the IPA vapor. The use of an external IPA vapor source can also contribute to substrate processing control system complexity, e.g., build up of contaminants in the IPA source liquid.